I don't mean to scare anyone, but I want to remind everyone how important it is to drink tons of water, take it easy (no extreme hiking!), and one alcoholic drink is like 3 drinks up here at 10,000ft. On most every single wedding I've worked on, at least one person is either sick in bed at the hotel room or is in the hospital with altitude sickness. On a few weddings even the bride has gotten sick, which is horrible and not something you would ever think of when your planning your dream mountain wedding.
Its not only the older or out of shape people who get altitude sickness either, its random and maybe a doctor or google has an answer for that but I've seen healthy 19 year olds get sick as well. When we have visitors up here in Summit County, sometimes we let them stay in Denver for a night to slowly acclimate there first before coming straight up to the mountains. Or for some people they have to start chugging water to get over-hydrated at home first before flying in to Colorado. We have homeopathic altitude pills available at the gas stations, mini oxygen bottles at Walmart and City Market, and you can rent an oxygen tank as well to share for your guests or a personal size to carry around. The best method if you plan to come here and be active right away is you can get a prescription drug from your doctor called Diamox that helps a lot when you start taking it a couple days before being in altitude.
Then there is the Oxygen bar in Breckenridge, Vitafusion and the health clinics where some people have to actually get fluid injections to feel better. Its happened to even locals here who go on vacation to sea level, if you're driving back home to the mountains and hit a point on I-70 where you start to get a headache, start drinking water! I just wanted to put this out there in hopes that if you didn't already know some of these tricks maybe it can help or prevent you or a guest from missing the big day because they feel like they have the worst hangover of their life, which is what altitude sickness feels like!